If This Comet Is What We Think It Is, It’s Unlike Any Object Humanity Has Ever Seen

DonSci/Tech2025-07-176640

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Comet 31/ATLAS is only the third interstellar object to ever be observed whizzing through the Solar System.

While its origins are uncertain, the comet is thought to come from a part of the Milky Way populated by some of the galaxy’s oldest stars.

While its age has not been confirmed, if 31/ATLAS really did originate in that region, it could be 7 billion years old (compared to the Solar System’s age of 4.6 billion).

Few rogue interstellar objects (ISOs) have entered our Solar System. ‘Oumuamua nearly broke the internet in 2017 when it streaked past, and Comet 2I/Borisov was caught trespassing by NASA Hubble Space Telescope in 2019. Now, we have another interloper.

Astronomers first observed Comet 31/ATLAS on July 1. Though its origins remain unknown, the researchers who have eyes on it think the flying hunk of ice, dust, and frozen gases came from what is known as the thick disk of the Milky Way. This region of ancient stars—most over 10 billion years old—is scattered in a layer above and below our galaxy’s thin disk (which is populated by younger stars).

If 31/ATLAS really was born in the thick disk, it might be the oldest comet ever spotted, at around 7 billion years old; that’s 3 billion years older than the Solar System. And the way it moves suspiciously up and down in the galactic plane is possibly indicative of an object having originated from and stayed in the disk for literal eons. If we’re lucky, it could give us an inside look at the formation of the early Milky Way.

This comet is thought to have originally been an icy planetesimal, one of the many smaller bodies that once merged to form planets. How it got to us today, however, is more debatable.

“31/ATLAS has physical differences to the first two ISOs detected, but its orbit and incoming velocity place it as a member of the Galactic population of ISOs,” the researchers said in a study, which was recently presented at the Royal Astronomical Society’s 2025 National Astronomy Meeting in Durham, England, and posted to the preprint server arXiv.

The research team employed a probability simulator that models stellar trajectories using the positions and velocities of every star ESA’s Gaia satellite observed between 2014 and 2025 (even the dead ones)—about a billion stars. This—combined with models of protoplanetary disk chemistry and dynamics within the galaxy—allowed them to predicted the velocities, ages, and chemical compositions of the Milky Way’s interstellar objects.

The experts might be able to determine properties of 31/ATLAS based on which stellar population it came from. Star systems—including our own Solar System—form from protoplanetary disks of gas and dust, and models assume that the properties of these disks are influenced by their stars. The models also suggest that ISOs stray from their stars early on, and disperse within the cold outer edges of a star system.

Another model used by the researchers predicts how the velocity of ISOs is related to their age, composition, and other properties inherited from the stellar populations from which they originate. This is how the team figured out a tentative age for 31/ATLAS, and that it probably does not come from the same star as the previous two interstellar objects we’ve seen.

There are still many missing pieces to the origin story of 31/ATLAS. Whether it trekked through space on a relatively undisturbed path, or had its orbit perturbed by different stars is unknown. Despite what has been suggested so far, the comet is still shrouded in gas, dust, and mystery. But because interstellar objects are thought to exist almost indefinitely, they could be carrying particles from just about every star that ever existed in the galaxy.

“ISOs provide the opportunity to gain evidence of the process of planetesimal formation and evolution from a host of galactic environments,” the researchers said. “Further observations of 31/ATLAS will allow us to [test our] assumptions.”